1. Field of the Invention
The present invention relates to a communication channel selection method and a mobile communication apparatus, and more particularly relates to an improvement of a communication channel selection method for selecting one radio communication channel from a plurality of radio communication channels provided by the system side, and also relates to an improvement of a mobile communication apparatus for selecting one radio communication channel from a plurality of radio communication channels provided by the system side.
2. Description of the Related Art
A mobile communication apparatus such as a portable telephone has a channel selection for selecting a communication channel since the apparatus is generally capable of using a plurality of radio communication channels provided by the communication system, whereby the mobile communication apparatus communicates through a communication channel selected by the channel selection section.
However, all of the communication channels are not always in a usable state. That is, even when a multiplicity of radio communication channels provided by the system side (for example, an earth station) are usable, the radio signal capable of being received by the mobile communication apparatus is limited under a particular circumstance. Accordingly, when determining a communication channel, the channel selection section is required to perform the channel search in such a manner that the communication channels capable of being selected are sequentially selected thereby finding a usable communication channel, that is, a channel capable of receiving the radio signal transmitted from the system side.
The communication channel is defined by frequency and time slot in the case of a portable telephone of the TDMA (time division multiple access) system. While, the communication channel is defined by frequency and spreading code in the case of a portable telephone of the CDMA (code division multiplex access) system. The information for defining the communication channel is called channel information. For, example, in the system employing the GSM (global system for mobile communications) system which is one of TDMA system, several hundred kinds of communication channels can be selected in accordance with combination of the frequencies and the time slots.
Accordingly, in the case where the channel switching is required, communication channels are sequentially selected from a channel list in which selectable communication channels are defined. And, the channel states of the selected communication channels are checked (channel search). When a communication channel in a good channel state is found, this communication channel is acquired. Thereafter, if the channel state of the acquired communication channel has degraded, the same channel search as the aforesaid channel search is executed again. In such a channel search, since communication channels are sequentially selected from the channel list, there is a case where a communication channel in a good channel state can be found through checking only a channel state of one communication channel, but there is also a case where a communication channel in a good channel state can be found for the first time through checking channel state of several hundreds of communication channels.
Although a time required for the checking operation of the channel state differs depending on the communication system, there is a case that the communication system required is a long time for checking operation. If a long time is required for the channel search, there arises a problem that a user can not immediately use the radio communication apparatus for communication, even if the user exists within a service area capable of receiving the radio signal from the system.
On the other hand, there has been proposed a portable telephone in which the communication channels within a channel list are grouped and the communication channels of a particular group are subjected to the checking of the channel states thereof in preference to other groups. In such a portable telephone, the channel list includes the communication channels (home channels) provided by the service company with which a user directly contracted and the communication channels (roaming channels) which are provided by other service companies and available for the same portable telephone by the roaming contract or the like, wherein the home channels and the roaming channels are memorized as separate groups. At the time of the channel search, the selection of the communication channel from the group of the home channels is performed in preference to the other groups, and the communication channel is selected from the group of the roaming channels only when none of all the home channels is in a good channel state.
In general, since the channel charge is cheaper in the home channels rather than the roaming channels, this portable telephone is arranged in such a manner that the communication channels of the channel list are grouped due to the economical reason. According to such grouping of the communication channels, although the time required for the channel search can be shortened within an area where the home channels can be acquired, it takes a long time for the channel search in an outside area where the home channels can not be acquired since only the roaming channels can be acquired.
Further, there has been proposed the system where a system (for example, an earth station) transmits a radio signal including information as to the communication channels of peripheral cells. For example, in the GSM system, the channel information on the radio signal (a cell corresponding to this radio signal is referred to as a serving cell) transmitted from the earth station monitored by the mobile communication apparatus includes the frequency information on cells (referred to as neighboring cells) adjacent to the serving cell. If the frequency information on such neighboring cells is obtained in advance, even when the mobile communication apparatus moves out of the serving area, the next communication channel can be acquired by the channel search in a shorter time by performing the channel search only as to such neighboring cells, compared with the case that the channel search is performed sequentially on the basis of the channel list.
However, as there is a communication system that requires several seconds for one channel checking cycle, even if the channel search is performed only as to the neighboring cells, it takes from several seconds to several tens of seconds to carry out channel switching.
Specifically, it has been a problem that it takes a long time to perform channel switching when there exist many neighboring cells. For example, in the case of a satellite communication system by way of a mobile satellite, the number of cells formed by the communication satellite is not constant, as the relative position of the communication satellite varies. For example, in the nearby area of immediately beneath the cross point of two satellite orbits, the two satellites shifting along their respective orbits independently form their own cells. For this reason, a plurality of cells are formed in a superimposed manner, causing a state in which there exist a plurality of neighboring cells, so that the time required for the channel switching becomes long.
The present invention is provided for solving the problems aforementioned, and it is an object of the present invention to provide a communication terminal apparatus capable of selecting in a short time a desired communication channel at the time of channel switching, by storing the capture time of communication channels in advance.